The present invention relates to an improved method and apparatus for tracing coaxial cables. Specifically, the present invention is useful for tracing a single coaxial cable within a group of coaxial cables.
A single coaxial cable includes a center conducting wire which is normally insulated from and surrounded by a braided outer conductor. Typical coaxial cable systems involve numerous coaxial cables, the outer conductors of which are often connected to a common ground. In many applications, including modern telecommunications systems, a bundle of as many as 50 coaxial cables may be employed. These cable bundles may extend a half mile in length. Often, for example, to locate and repair a fault, it becomes necessary to monitor the performance of a given cable or, sequentially, a number of cables within a bundle. In such instances, a technician may be required to locate or trace a single cable within a bundle of cables.
Currently, equipment which was developed for tracing twisted pair cables is also used for tracing coaxial cables. In the industry, this equipment is commonly referred to as a "tag toner." Tag toners employ an audio frequency generator to apply an audio frequency voltage signal to the center conductor (relative to the outer conductor) of a coaxial cable. At one end of the coaxial cable, a high impedance amplifier detects the applied voltage signal, amplifies it and outputs the amplified audio signal to a speaker. An operator identifies the cable of interest by listening for an audio tone at the opposite end of the cable to which the audio signal was applied. Only at the ends of the cable are both the center and outer conductors accessible to the amplifier; thus, it is only at the cable's end that the amplifier can detect the voltage signal applied between the center and outer conductors.
Accordingly, voltage tracing methods are only capable of identifying the end of a coaxial cable and are useless for midspan identification. In voltage tracing systems, midspan identification of the specific cable to which the voltage has been applied is impossible because the voltage tends to bleed over to the other lines in the bundle, making the cable of interest indistinguishable from the other cables, except at the cable's end.
Voltage tracing systems can be used to apply a voltage between the outer conductor of a coaxial cable and earth ground, which allows an amplifier to sense voltage midspan along the cable. However, this voltage cannot be used to differentiate one cable from another because in coaxial cable systems the outer conductors of each cable in the bundle are generally all connected to a common ground (shorted together). Thus, any voltage relative to earth ground which is detected on the outer conductor of one cable will appear on the outer conductor of each cable, making it impossible to trace or identify an individual cable.
A current tracing system exists which can be used only for tracing faulted twisted wire pairs. This system is commonly known a "Breakdown Set"--a bulky, expensive system which welds the twisted wire pair together at the fault and then drives a very large current (3 A peak to peak) through the welded pair. An operator then uses a current sensor and amplifier to trace the magnetic field generated by the applied current to the fault location. The Breakdown Set is useful only on faulted cables, and permanently shorts the cable under test at the fault location (i.e., it is a destructive tracing method).
Because the Breakdown Set is intended for welding faulted cables, it is not practical for use with coaxial cables which have much thicker insulation and are therefore much more difficult to weld. Because the Breakdown Set operates in the audio frequency range, a one and a half amp current would have to be applied to the cable under test in order to generate a detectable signal. Even a one and a half amp current will provide only weak signal. Further, application of a one and a half amp current would damage any other equipment (such as VCR's etc.) which may be connected to the cable under test.
Thus, there is a need in the art for a method and apparatus that will allow an operator to identify a single coaxial cable at any point along its length.
Accordingly, it is an object of the present invention to provide a method and apparatus that will allow a single coaxial cable within a cable bundle to be identified at any point along the cable's length.
It is a further object of the present invention to provide a unique method and apparatus for combining both current and voltage tracing, thereby obtaining the benefits of both systems.
A further object of the invention is to provide an apparatus and method that advantageously employs the imbalance ordinarily occurring in coaxial cable systems.
Yet another object of the invention is to provide a compact and inexpensive apparatus for tracing and identifying a specific coaxial cable within a cable bundle, at any point along the cable's length.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiment, the appended claims and the accompanying drawings, in which:
FIG. 1 is a schematic diagram depicting prior art voltage tracing as applied to a coaxial cable system.
FIG. 2 is a schematic diagram depicting a preferred embodiment for current tracing as applied to a coaxial cable system.
FIG. 3 is a block diagram depicting the apparatus of the present invention in a preferred embodiment.
FIG. 4 is a timing diagram describing the preferred operation of the apparatus of FIG. 3.